Method and plant for manufacturing cement clinker

ABSTRACT

Described is a method for manufacturing cement clinker by which method cement raw meal is preheated and burned in a plant comprising a cyclone preheater ( 1 ) and a kiln ( 5 ). The method is peculiar in that that at least a portion of the raw meal is extracted from the cyclone preheater ( 1 ), that this raw meal is introduced into a separate unit ( 21 ) in which it is given a retention time under oxidating conditions provided by means of a gas stream for forming SO2 and for expelling organic carbon, that the formed SO2 and the expelled organic carbon are subsequently discharged from the separate unit ( 21 ) entrained in the gas stream for further treatment in a subsequent process stage, and that the raw meal is reintroduced into the cyclone preheater ( 1 ). Hereby is obtained an effective reduction of the VOC, CO as well as the SO2 emission without necessitating utilization of additional energy for heating. By giving the extracted and partially preheated raw meal a retention time under oxidating conditions separate from the cyclone preheater it is obtained that sulphide will oxidate into SO2 and that organic carbon is expelled from the raw meal, so that the thus formed SO2 and the thus expelled organic carbon can be entrained in a separate, relatively small gas stream and subjected to subsequent treatment in the optimum manner.

The present invention relates to a method for manufacturing cementclinker by which method cement raw meal is preheated and burned in aplant comprising a cyclone preheater and a kiln. The invention relatesspecifically to a method for reducing the emission of SO₂, CO andvolatile organic compounds (hereinafter referred to as VOC) from such aplant. The invention also relates to a plant for carrying out themethod.

Plants of the aforementioned kind for manufacturing cement clinker arewell known from the literature.

The emission of SO₂, CO and VOC from such kiln plants for manufacturingcement clinker emanate primarily from the raw materials which are beingused as described in further details in the following text. The heatingof the raw meal in the cyclone preheater is done by direct contact withhot exhaust gases according to the counterflow principle, whereby theformed SO₂ and CO and the expelled VOC are immediately captured by theexhaust gas stream, and thus leave the cyclone preheater together withthe exhaust gas stream in the form of emission. For different reasons,the three types of emission into the atmosphere are undesirable.

The cement raw materials often contain minerals such as pyrite andmarcasite. The sulphide in the pyrite (FeS₂) is converted in the cyclonepreheater at temperatures around 525° C., causing SO₂ to be formed.Measurements performed at an operating cement plant have thus shown thatvirtually all the sulphide contained in the raw meal feed will still bepresent in the raw meal when it leaves the first cyclone stage at atemperature around 370° C., whereas the sulphide content in the raw mealleaving the second cyclone stage at a temperature around 550° C. will beapproximately half as high. So, at the cement plant in question nearlyone half of the sulphide contained in the raw materials will escape fromthe preheater in the form of SO₂ as a result of the oxidation processwhich takes place in the second cyclone stage. A known method forreducing the SO₂ level involves application of an absorbent in the formof CaO, Ca (OH)₂ or other basic components in the cyclone preheater at alocation after, viewed in the direction of flow of the exhaust gases,SO₂ is formed so that SO₂ can be bound in the raw material in the formof sulphite which is transformed into sulphate at a subsequent processstage. One significant disadvantage of this known method is that it willusually be necessary to apply an excess amount of absorbent, thus makingit a relatively expensive method.

Also, the cement raw materials will frequently contain organic carbonwhich is expelled substantially from the raw meal in the form of CO andVOC during the preheating process in the cyclone preheater and beingdischarged in unburned form together with the exhaust gas stream. Thisis confirmed by studies which indicate that certain types of VOC areexpelled substantially within a relatively narrow temperature span. Onetype of VOC is thus expelled substantially within a temperature spanranging from 300 to 500° C., whereas another type is expelledsubstantially within a temperature span ranging from 450 to 650° C.Other additional types of VOC are expelled over greater temperaturespans. In a traditional cyclone preheater the aforementioned temperaturespan will typically occur in the 1st and 2nd cyclone stage, and,respectively, the 2nd and 3rd cyclone stage, dependent on whether thecyclone preheater is a 4-stage or 5-stage unit and also somewhatdependent upon the efficiency of the other elements of the kiln system.Several methods are known for the subsequent treatment of the exhaustgases for removing VOC from the exhaust gases. A known method comprisesthe steps, that the exhaust gases from the preheater are reheated in aheat exchange unit, that VOC are burned while fuel is simultaneouslyadded, and that the exhaust gases are subsequently cooled in the heatexchange unit. From the viewpoint of energy consumption this is not anoptimum solution, and the apparatus for carrying out the method alsoinvolves quite substantial investment costs.

In addition, from Danish patent application No. PA 2001 00009 is known amethod by which raw meal is extracted from the preheater and heated in aseparate heating unit for forming SO₂ and for expelling VOC. Accordingto the known method, the formed SO₂ is subsequently brought into contactwith an absorbent, the expelled VOC is burned off, and the raw meal isreintroduced into the cyclone preheater. The disadvantage of this knownmethod is primarily that the energy consumption will be relatively high.

It is the objective of the present invention to provide a method as wellas a plant for manufacturing cement clinker by which a cheap andeffective reduction of SO₂, CO and VOC emission can be achieved withoutany significant impact on the efficiency level of the cyclone preheater.

This is achieved by a method of the kind mentioned in the introductionand being characterized in

-   -   that at least a portion of the raw meal is extracted from the        cyclone preheater,    -   that this raw meal is introduced into a separate unit in which        it is given a retention time under oxidating conditions provided        by means of a gas stream for forming SO₂ and for expelling        organic carbon,    -   that the formed SO₂ and the expelled organic carbon are        subsequently discharged from the separate unit entrained in the        gas stream for further treatment in a subsequent process stage,        and    -   that the raw meal is reintroduced into the cyclone preheater.

Hereby is obtained an effective reduction of the VOC, CO as well as theSO₂ emission without necessitating utilization of additional energy forheating. By giving the extracted and partially preheated raw meal aretention time under oxidating conditions separate from the cyclonepreheater it is obtained that sulphide will oxidate into SO₂ and thatorganic carbon is expelled from the raw meal, so that the thus formedSO₂ and the thus expelled organic carbon can be entrained in a separate,relatively small gas stream and subjected to subsequent treatment in theoptimum manner. As will be described in further details in the followingtext, studies carried out by the applicant have surprisingly indicatedthat a significant oxidation of sulphide into SO₂ and a certainexpulsion of organic carbon will occur even if the temperature is keptconstant and below that at the location in the cyclone preheater wheremost of the SO₂ release is otherwise taking place. The studies have alsoshown that the rate at which these processes are taking place executeddepend on the temperature, and that the rate will be increased in stepwith rising temperatures.

The plant for carrying out the method according to the invention ischaracterized in that it comprises means for extracting at least aportion of the raw meal from the cyclone preheater, separate means forgiving this raw meal a retention time under oxidating conditions andthereby ensuring oxidation, by means of a gas stream, of sulphidecontained in this raw meal for the formation of SO₂ and for theexpulsion of organic carbon, means for discharging the formed SO₂ andthe expelled organic carbon from the separate unit entrained in the gasstream for further treatment in a subsequent process stage, and meansfor reintroducing the raw meal into the cyclone preheater.

Further characteristic features of the plant will be apparent from thedetailed description provided in the text below.

It is preferred that all of the raw meal is extracted from the cyclonepreheater for oxidation in the separate unit.

Up to this point in time, conventional wisdom has held that when the rawmaterials contain sulphurous components, SO₂ will be formed within arelatively small temperature span around 525° C. The studies referred toabove and described in further details in the following text have,however, quite surprisingly indicated that a significant oxidation ofsulphide into SO₂ will occur even at lower temperatures if only thenecessary time is allocated for the process. The studies have thus shownthat the formation of SO₂ may occur even at a temperature of 350° C.,and according to the invention the raw meal may thus be extracted fromthe cyclone preheater at a temperature ranging between 350° C. and 525°C. In order to limit the necessary time of retention for the extractedraw meal in the separate unit and thus its capacity, it is preferredthat the raw meal is extracted from the cyclone preheater at atemperature within the range of 400° C. and 500° C. The studies carriedout have indicated that at temperatures higher than 525° C., SO₂ will beformed so rapidly that virtually all of the sulphide has been convertedinto SO₂ before the raw meal is extracted from the preheater.

In principle, the raw meal can be given any retention time in theseparate unit which is necessary to attain the desired SO₂ formation atthe temperature in question. In actual practice, the temperature of theextracted raw meal will be the main determinant for the duration of theretention time, which is necessary. According to the invention theretention time in the separate unit may be selected at random, but,often, it should advantageously be within the range of 10 to 200seconds. However, it is preferred that a maximum limit of 100 seconds isapplied.

The temperature in the separate unit can be kept substantially constantduring the oxidation process, but it may also be varied, for examplethrough regulation of the temperature of the gas stream introduced intothe separate unit. If it is desirable to increase the rate of oxidation,the temperature in the separate unit may thus be elevated by introducinga hotter gas stream.

In principle, organic carbon is expelled from the raw meal across theentire temperature span in the cyclone preheater, which in the case ofthe raw meal ranges from a temperature of less than 100° C. at the topof the cyclone preheater to a temperature close to 830° C. at the bottomof the cyclone preheater. Therefore, the method according to theinvention will only have the capability to expel a portion of the totalamount of organic carbon. The temperature at which the raw meal is to beextracted will therefore depend primarily on the temperature at whichthe maximum reduction of the SO₂ level is achieved. The studiespreviously referred to in the text have indicated variations in thepattern for expelling different types of organic carbon. Prior toimplementation of the method according to the invention at a givencement plant it will, therefore, be advantageous to conduct specificinvestigations of the raw materials utilized in order to determineexactly their content of different types of organic carbon and furtherto determine how these are expelled as a function of the temperature. Insome cases, where consistent with the SO₂ reduction, it is, therefore,preferred that the raw meal is extracted from the cyclone preheater at atemperature of less than 450° C.

In principle, the oxidation of the extracted raw meal in the separateunit can be done in any suitable manner, however, a smalleroxygen-containing gas stream must be led through the compartment foroxidation of sulphide and organic carbon and for the removal of SO₂ andexpelled organic carbon. Studies have indicated that the optimum oxygenpercentage for removing SO₂ is approximately 5 per cent.

The separate unit may be configured in any suitable manner. The separateunit may comprise any type of receptacle or conveying mechanism for bulkmaterials, which will be able to provide a sufficient retention time forthe raw meal and ensure a sufficient mixing of the raw meal and theoxygen-containing gas stream. For example, the separate unit may beconfigured as a rotary drum in which the extracted raw meal and theoxygen-containing gas stream are introduced via inlets located at eitherend of the rotary drum, passed through the rotary drum incounter-current flow and also discharged from opposite ends.Furthermore, it is desirable that the unit or plant comprises means forensuring that the raw meal after its extraction from the unit will bephysically located at, or can be routed to, the level which is necessaryfor it to be reintroduced at the designated location into the cyclonepreheater.

The formed SO₂ which is discharged from the separate unit entrained inthe gas stream can be separately treated in, for example, a wet scrubberof known operating principle where SO₂ on reaction with CaCO₃ and H₂Owill be transformed into gypsum on the form CaSO₄.2H₂O, and from wherethe cleaned gas can be released to the environment. The necessary CaCO₃may be contained in the dust carried along from the separate oxidationunit or it may be supplied in the form of fresh raw meal. Due to thefact that the gas stream through the separate oxidation unit is smallrelative to the gas stream through the cyclone preheater, the wetscrubber for this purpose may also be relatively small. The waterconsumption will also be relatively small. The gypsum generated in thewet scrubber may advantageously be used at the cement mill plant insubstitution for some of the ordinary gypsum. In this way a significantamount of sulphur may be bypassed the kiln system of the cement plant,thereby reducing the frequently occurring problems in respect ofclogging and obstruction in the kiln system.

The formed SO₂ which is discharged from the separate unit entrained inthe gas stream may alternatively be introduced into the cyclonepreheater at a location where a sufficient amount of absorbent in theform of CaO and/or other basic components is present, which willtypically be at the lower end of the cyclone preheater. In plants wherethe cyclone preheater comprises a calciner it is preferred that theformed SO₂ is introduced into the said calciner.

The expelled organic carbon can be burned separately or alternatively itmay be reintroduced at a location in the cyclone preheater where thetemperature is at least 700° C., which will typically be at the lowerend of the preheater. In plants where the cyclone preheater comprises acalciner it is preferred that the expelled organic carbon is introducedinto the said calciner.

In principle, the extracted and separately oxidated raw meal can bereintroduced into the cyclone preheater at any location. However, itshould preferably be introduced immediately after the point of raw mealextraction, viewed in the direction of flow of the raw meal. In otherwords, it is preferred that the separately oxidated raw meal isintroduced into the cyclone preheater at the first cyclone stage afterthe cyclone stage from which it was extracted.

The invention will now be described in further details with reference tothe drawing, being diagrammatical, and in which

FIG. 1 shows a graphic representation of the formation of SO₂ as afunction of the time at different temperatures,

FIG. 2 shows a first example of a plant according to the invention, and

FIG. 3 shows a second example of a plant according to the invention.

Seen in FIG. 1 are graphs a-d for the formation of SO₂ as a function ofthe time at the temperatures 350, 375, 400 and 500° C. The illustratedgraphs are a direct result of a series of tests which have beenconducted by the applicant. The tests have been conducted in a fixed bedreactor according to the method described in the following. A materialsample was preheated to a desired temperature in an inert hot gasconsisting of pure N₂. The hot exit gas from the inert heating processwas mixed with O₂ in order to oxidate any evaporated sulphur into SO₂.Following inert heating during a period of 240 seconds, O₂ was added tothe N₂ stream before the stationary bed. This procedure was followed tomake certain that there would not be any significant oxidation of thematerial sample during the relatively slow heating process and that thesupply of O₂ would take place swiftly relative to the chemical reactionso that the rate of reaction could be studied at a given temperature andindependently of the heating process. The SO₂ content as a function ofthe time was measured throughout the test procedure. In FIG. 1 that partof the graphs where t is less than 0 shows the SO₂ formation whichoccurs in connection with the heating of the material in the inert gaswhereas the graphs for t greater than 0 show the SO₂ formation when thetemperature is kept constant and when O₂ is supplied. It clearly appearsfrom the figure that the formation of SO₂ is a very slow process duringthe inert heating process up to the point where t is equal to 0 and thatsubsequently, when O₂ is supplied when t is equal to 0 the formationrate is much faster. In particular the graphs b, c and d show that theconversion of sulphide into SO₂ takes place predominantly within theinitial 100-second period whereafter the graphs are levelled out,presumably trending towards maximum conversion which, in respect of thematerial being used during the test, will be approximately 30, 50 and 70percent, respectively, at the respective temperatures 375, 400 and 500°C. So, out of these three temperatures the immediate conclusion would bethat the optimum method would involve extraction of the raw material ata temperature around 500° C. since the highest degree of conversion isachieved at this temperature. However, it should be noted that inconnection with the heating to a level of 500° C. in the inert gas, aquite significant amount of SO₂ will be formed which in actual practicewill not be transferred to the separate unit. So, the optimumtemperature for extracting the material in question seems to be withinthe range of 400° to 500° C.

In FIGS. 2 and 3 are seen two examples of plants according to theinvention. Both of the plants shown comprise a cyclone preheater 1, arotary kiln 5 and a clinker cooler 7. The cyclone preheater 1 comprisesfour cyclone stages a-d, a calciner 3 and a separation cyclone 4. Thecyclone preheater 1 may comprise fewer as well as more than the fourcyclones indicated. Raw meal from a not shown raw mill plant isintroduced into the cyclone preheater via one or several inlets 9 andpreheated in a counter-current arrangement with exhaust gases whereafterit is separated from the cyclone preheater in the cyclone d and directedto the calciner 3 in which it is calcined. From the bottom outlet of theseparation cyclone 4, the calcined raw meal is then directed via a duct8 to the rotary kiln 5 in which it is burned into cement clinker whichis subsequently cooled in the clinker cooler 7. The exhaust gases fromthe rotary kiln 5 and the calciner 3 are drawn through the cyclone 4 andthen up through the cyclone preheater by means of a fan 6. Tertiary airfrom the clinker cooler 7 is introduced via a duct 11 into the calciner3.

According to the invention at least some of the raw meal is extractedfrom the cyclone preheater 1 with a view to subjecting it to oxidationin a separate unit 21 in which the raw meal is introduced via a duct 15.In order for the separate oxidation of the raw meal to have anysignificant effect upon the formation of SO₂ and the expulsion oforganic carbon, the raw meal must of course be extracted from thepreheater before the majority of the sulphide content has beentransformed into SO₂ and/or before the content of organic carbon hasbeen expelled from the material. In instances where it is only desirableto carry out separate oxidation of some of the raw meal, it can beextracted from the raw meal flow from the bottom outlet of the selectedcyclone stage via, for example, a splitter gate 13.

The separate unit 21 shown in FIGS. 2 and 3 comprises a rotary drum 21.A gas stream is introduced via an inlet 27 at one end of the rotary drumand the extracted raw meal is introduced via an inlet 29 at the oppositeend, causing the mixing of the raw meal and the gas stream to beeffected in a counterflow arrangement. The raw meal is extracted fromthe other end of the rotary drum 21 via an outlet 22 and directed via aduct 26 and transport means, if any, back to the cyclone preheater 1into which it is reintroduced via an inlet 28 which is locatedimmediately after the point at which it was extracted, viewed in thedirection of flow of the raw meal.

In the embodiment shown in FIG. 2 the gas stream containing SO₂ and theexpelled organic carbon is led from an outlet 29 in the rotary drum 21via a duct 17 to the calciner 3 in which all organic carbon is burnedoff and SO₂ is absorbed under optimum temperature conditions by reactionprimarily with CaO.

In the embodiment shown in FIG. 3 the gas stream containing SO₂ and theexpelled organic carbon is led from the outlet 29 in the rotary drum 21via the duct 17 to a wet scrubber 31 in which it is cleaned according toknown principles where SO₂ by reaction with CaCO₃ and H₂O will betransformed into gypsum on the basis of the form CaSO₄.2H₂O, and fromwhere the cleaned gas can be released to the environment, possibly via aunit 33 for burning CO and VOC.

In connection with the implementation of the invention at an existingcement plant it will often be necessary to set the temperature in thepreheater at a level which will allow the raw meal to be extracted atthe desired temperatures. This can be done in a number of ways. If it isdesirable to lower the temperature at the specific location in thepreheater where the raw meal is to be extracted for the separateoxidation, it will be possible to introduce, for example, atmosphericair at an appropriate location. However if it is desirable to raise thetemperature at the specific point of extraction, the raw meal feed may,for example, be split up and a smaller quantity of raw meal may bebypassed. Also, the temperature can be adjusted by controlling thequantity of raw meal being extracted for the separate oxidation. Othermeans of regulation consist in a modification or regulation of theseparation efficiency of the preheater cyclones.

In actual practice it may be necessary to regulate the capacity of theseparate unit 21 and in the case of a rotary drum this may be done bychanging its rotational speed. An increase in the rotational speed ofthe rotary drum will lead to a reduction in the retention time of theraw meal in the drum, entailing a corresponding reduction in the amountof SO₂ being formed and the organic carbon being expelled. In order tocompensate for any such reduction, heat may be supplied to the separateunit 21, possibly by introducing a partial flow stream of the hot airfrom the clinker cooler 7 into the unit 21 via the inlet 27.

1. A method for manufacturing cement clinker comprising: preheating andburning cement raw meal in a plant comprising a cyclone preheater and akiln; extracting at least a portion of the raw meal from the cyclonepreheater; introducing the raw meal into a separate unit in which theraw meal is given a retention time under oxidating conditions providedby means of a gas stream for forming SO2 and for expelling organiccarbon; discharging the formed SO2 and the expelled organic carbon fromthe separate unit entrained in a gas stream for further treatment in asubsequent process stage; and reintroducing the raw meal into thecyclone preheater.
 2. The method according to claim 1, wherein all ofthe raw meal is extracted from the cyclone preheater for oxidation inthe separate unit.
 3. The method according to claim 1, wherein the rawmeal is extracted from the cyclone preheater at a temperature between350° C. and 525° C.
 4. The method according to claim 2, wherein the rawmeal is extracted from the cyclone preheater at a temperature between350° C. and 525° C.
 5. The method according to claim 1, wherein the rawmeal is extracted from the cyclone preheater at a temperature between400° C. and 500° C.
 6. The method according to claim 2, wherein the rawmeal is extracted from the cyclone preheater at a temperature between400° C. and 500° C.
 7. The method according to claim 1, wherein thetemperature in the separate unit is kept substantially constant duringthe oxidation process.
 8. The method according to claim 2, wherein thetemperature in the separate unit is kept substantially constant duringthe oxidation process.
 9. The method according to claim 1, wherein theraw meal is given a retention time in the separate unit within the rangeof 10 to 200 seconds.
 10. The method according to claim 2, wherein theraw meal is given a retention time in the separate unit within the rangeof 10 to 200 seconds.
 11. The method according to claim 1, wherein theraw meal is given a retention time in the separate unit within the rangeof 10 to 100 seconds.
 12. The method according to claim 2, wherein theraw meal is given a retention time in the separate unit within the rangeof 10 to 100 seconds.
 13. The method according to claim 1, wherein thethat the formed SO2 and the expelled organic carbon, and which isdischarged from the separate unit, is introduced into a calciner of thecyclone preheater.
 14. The method according to claim 2, wherein the thatthe formed SO2 and the expelled organic carbon, and which is dischargedfrom the separate unit, is introduced into a calciner of the cyclonepreheater.
 15. The method according to claim 1, wherein the extractedand separately oxidated raw meal is introduced into the cyclonepreheater immediately after the point where it was extracted, viewed inthe direction of flow of the raw meal.
 16. The method according to claim2, wherein the extracted and separately oxidated raw meal is introducedinto the cyclone preheater immediately after the point where it wasextracted, viewed in the direction of flow of the raw meal.
 17. A plantfor manufacturing cement clinker comprising a cyclone preheater; a kiln,means for extracting at least a portion of the raw meal from the cyclonepreheater; separate means for giving the raw meal a retention time underoxidating conditions and thereby ensuring oxidation by means of a gasstream of sulphide contained in this raw meal for the formation of SO2and for the expulsion of organic carbon; means for discharging theformed SO2 and the expelled organic carbon from the separate unitentrained in a gas stream for further treatment in a subsequent processstage; and means for reintroducing the raw meal into the cyclonepreheater.
 18. The plant according to claim 17, further comprising a wetscrubber for treatment of the formed SO2, which is discharged from theseparate unit entrained in the gas stream.